1. Field of the Invention
The invention relates to a liquid crystal compound, a liquid crystal composition and a liquid crystal display device. More specifically, the invention relates to a five-ring 2,3-difluorobenzene derivative that is liquid crystalline, a liquid crystal composition including this compound and having a nematic phase, and a liquid crystal display device containing this composition.
2. Related Art
A liquid crystal display device typified by a liquid crystal display panel, a liquid crystal display module and so forth utilizes optical anisotropy, dielectric anisotropy and so forth, which are possessed by a liquid crystal compound (in this invention the liquid crystal compound is used as a generic term for a compound having a liquid crystal phase such as a nematic phase or a smectic phase, and a compound having no liquid crystal phases but useful as a component of a liquid crystal composition). As an operating mode of this liquid crystal display device, a variety of modes are known, such as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode.
In the operating mode, the ECB mode, the IPS mode, the VA mode and so forth utilize homeotropic orientation of liquid crystal molecules, and it is known that in particular the IPS mode and the VA mode are able to improve a limited viewing angle that is a disadvantage of a conventional display mode such as the TN mode and the STN mode.
A variety of liquid crystal compounds in which hydrogen on the benzene ring had been replaced by fluorine have conventionally been studied as a component of a liquid crystal composition having negative dielectric anisotropy which can be used for liquid crystal display devices having these operating modes.
For example, the compounds (A) and (B), in which hydrogen on the benzene ring had been replaced by fluorine, were studied. However, these compounds did not have such a large negative dielectric anisotropy that satisfied market demand.
The compound (C) having a fluorine-substituted benzene ring was studied. However, this compound did not have such a large negative dielectric anisotropy that satisfied market demand.
The quarterphenyl compound (D) having two fluorine-substituted benzene rings was studied (see patent document No. 1). However, this compound had a quite high melting point and a poor compatibility. The compound did not have such a large negative dielectric anisotropy that satisfied market demand.
The compound (E) having an ethylene bonding group and three fluorine-substituted benzene rings was studied (see patent document No. 2). However, this compound (E) had a high melting point and a poor compatibility. The compound did not have such a large negative dielectric anisotropy that satisfied market demand.
The five-ring compound (F) having a bonding group and a fluorine-substituted benzene ring located in the middle of the molecule was studied (see patent document No. 3). However, the compound (F) did not have such a large negative dielectric anisotropy that satisfied market demand.

Patent document No. 1: EP 1,346,995 A1.
Patent document No. 2: WO 98/23564 A (1998).
Patent document No. 3: WO 2009/034867 A.
Accordingly, there are still subjects to be solved even in a liquid crystal display device having an operating mode such as an IPS mode or a VA mode, and, for example, an improvement of the response speed, an improvement of the contrast and a decrease of the driving voltage are expected.
A display device operated by the IPS mode or the VA mode described above mainly contains a liquid crystal composition having negative dielectric anisotropy, and a liquid crystal compound included in this liquid crystal composition is required to have the following characteristics shown in items (1) to (8), in order to improve the characteristics of the composition. That is to say:
(1) being chemically stable and physically stable,
(2) having a high clearing point (transition temperature between a liquid crystal phase and an isotropic phase),
(3) having a low minimum temperature of a liquid crystal phase (a nematic phase, a smectic phase and so forth), especially of the nematic phase,
(4) having a small viscosity,
(5) having a suitable optical anisotropy,
(6) having a large negative dielectric anisotropy,
(7) having a suitable elastic constant K33 (K33: a bend elastic constant), and
(8) having an excellent compatibility with other liquid crystal compounds.
A voltage holding ratio can be increased when a composition including a chemically and physically stable liquid crystal compound, as described in item (1), is used for a display device.
The temperature range of a nematic phase is wide in a composition that includes a liquid crystal compound having a high clearing point or a low minimum temperature of a liquid crystal phase as described in items (2) and (3), and thus the device can be used in a wide temperature range.
When a composition that includes a compound having a small viscosity as described in item (4) and a compound having a large elastic constant K33 as described in item (7) is used for a display device, the response speed can be improved. When a composition that includes a compound having a suitable optical anisotropy as described in item (5) is used for a display device, an improvement of the contrast in the display device can be expected. A device requires compositions having a small to large optical anisotropy, depending on the design of the device. Recently, a method for improving the response speed by means of a decreased cell thickness has been studied, whereby a liquid crystal composition having a large optical anisotropy is also required.
When a liquid crystal compound has a large negative dielectric anisotropy, the threshold voltage of the liquid crystal composition including this compound can be decreased. Hence, the driving voltage of a display device can be decreased and the electric power consumption can also be decreased, when the display device contains a composition that includes a compound having a large dielectric anisotropy as described in item (6). The driving voltage of a display device can be decreased and the electric power consumption can also be decreased, when a display device contains a composition that includes a compound having a suitable elastic constant K33 as described in item (7).
A liquid crystal compound is generally used in the form of a composition prepared by mixing it with many other liquid crystal compounds in order to exhibit characteristics that are difficult to be attained by a single compound. Accordingly, it is desirable that a liquid crystal compound used for a display device has an excellent compatibility with other liquid crystal compounds and so forth, as described in item (7). Since the display device may also be used in a wide temperature range including a lower temperature than the freezing point, the compound that exhibits an excellent compatibility even at a low temperature may be desirable.
As a result of earnest studies in consideration of these subjects, the inventors have found that a five-ring liquid crystal compound having two benzene rings at each end, in which hydrogen at the 2- and 3-positions is replaced by fluorine, has a high stability to heat, light and so forth, a wide temperature range of a nematic phase, a small viscosity, a large optical anisotropy and a suitable elastic constant K33, and further has a large negative dielectric anisotropy and an excellent compatibility with other liquid crystal compounds. The inventors have also found that a liquid crystal composition that includes the compound has a high stability to heat, light and so forth, a small viscosity, a large optical anisotropy, a suitable elastic constant K33, a suitable and large dielectric anisotropy, a low threshold voltage, a high maximum temperature of a nematic phase and a low minimum temperature of a nematic phase. The inventors have further found that a liquid crystal display device that contains the composition has a short response time, low electric power consumption, a low driving voltage, a large contrast ratio and a wide temperature range in which the device can be used. Thus, the inventors have completed the invention.